Considerable progress has been made in
the understanding of climate change1
science since 1990 and new data and analyses have become available.

1. Greenhouse gas concentrations
have continued to increase

Increases in greenhouse gas concentrations
since pre­industrial times (i.e., since about 1750) have led to a positive
radiative forcing2 of climate, tending
to warm the surface and to produce other changes of climate.

The growth rates of CO2, CH4 and N2O concentrations
were low during the early 1990s. While this apparently natural variation
is not yet fully explained, recent data indicate that the growth rates
are currently comparable to those averaged over the 1980s.

The direct radiative forcing of the long­lived
greenhouse gases (2.45 Wm­2) is due primarily to increases in the concentrations
of CO2 (1.56 Wm­2), CH4 (0.47 Wm­2) and N2O (0.14 Wm­2) (values
for 1992).

Many greenhouse gases remain in the atmosphere
for a long time (for CO2 and N2O, many decades to centuries), hence they
affect radiative forcing on long time­scales.

The direct radiative forcing due to the
CFCs and HCFCs combined is 0.25 Wm­2. However, their net radiative
forcing is reduced by about 0.1 Wm­2 because they have caused stratospheric
ozone depletion which gives rise to a negative radiative forcing.

Growth in the concentration of CFCs, but
not HCFCs, has slowed to about zero. The concentrations of both CFCs and
HCFCs, and their consequent ozone depletion, are expected to decrease substantially
by 2050 through implementation of the Montreal Protocol and its Adjustments
and Amendments.

At present, some long­lived greenhouse
gases (particularly HFCs (a CFC substitute), PFCs and SF6) contribute little
to radiative forcing but their projected growth could contribute several
per cent to radiative forcing during the 21st century.

If carbon dioxide emissions were maintained
at near current (1994) levels, they would lead to a nearly constant rate
of increase in atmospheric concentrations for at least two centuries, reaching
about 500 ppmv (approaching twice the pre­industrial concentration
of 280 ppmv) by the end of the 21st century.

A range of carbon cycle models indicates
that stabilization of atmospheric CO2 concentrations at 450, 650 or 1000
ppmv could be achieved only if global anthropogenic CO2 emissions drop
to 1990 levels by, respectively, approximately 40, 140 or 240 years from
now, and drop substantially below 1990 levels subsequently.

Any eventual stabilized concentration
is governed more by the accumulated anthropogenic CO2 emissions from now
until the time of stabilization than by the way those emissions change
over the period. This means that, for a given stabilized concentration
value, higher emissions in early decades require lower emissions later
on. Among the range of stabilization cases studied, for stabilization at
450, 650 or 1000 ppmv, accumulated anthropogenic emissions over the period
1991 to 2100 are 630 GtC3, 1030 GtC and
1410 GtC, respectively (approximately 15% in each case). For comparison
the corresponding accumulated emissions for IPCC IS92 emission scenarios
range from 770 to 2190 GtC.

Stabilization of CH4 and N2O concentrations
at today's levels would involve reductions in anthropogenic emissions of
8% and more than 50% respectively.

There is evidence that tropospheric ozone
concentrations in the Northern Hemisphere have increased since pre­industrial
times because of human activity and that this has resulted in a positive
radiative forcing. This forcing is not yet well characterized, but it is
estimated to be about 0.4 Wm­2 (15% of that from the long­lived
greenhouse gases). However, the observations of the most recent decade
show that the upward trend has slowed significantly or stopped.

2. Anthropogenic aerosols
tend to produce negative radiative forcings

Tropospheric aerosols (microscopic airborne
particles) resulting from combustion of fossil fuels, biomass burning and
other sources have led to a negative direct forcing of about 0.5 Wm­2,
as a global average, and possibly also to a negative indirect forcing of
a similar magnitude. While the negative forcing is focused in particular
regions and subcontinental areas, it can have continental to hemispheric
scale effects on climate patterns.

Locally, the aerosol forcing can be large
enough to more than offset the positive forcing due to greenhouse gases.

In contrast to the long­lived greenhouse
gases, anthropogenic aerosols are very short­lived in the atmosphere,
hence their radiative forcing adjusts rapidly to increases or decreases
in emissions.

3. Climate has changed
over the past century

At any one location, year­to­year
variations in weather can be large, but analyses of meteorological and
other data over large areas and over periods of decades or more have provided
evidence for some important systematic changes.

Global mean surface air temperature has
increased by between about 0.3 and 0.6°C since the late 19th century;
the additional data available since 1990 and the re­analyses since
then have not significantly changed this range of estimated increase.

Recent years have been among the warmest
since 1860, i.e., in the period of instrumental record, despite the cooling
effect of the 1991 Mt Pinatubo volcanic eruption.

Night­time temperatures over land
have generally increased more than daytime temperatures.

Regional changes are also evident. For
example, the recent warming has been greatest over the mid­latitude
continents in winter and spring, with a few areas of cooling, such as the
North Atlantic ocean. Precipitation has increased over land in high latitudes
of the Northern Hemisphere, especially during the cold season.

Global sea level has risen by between
10 and 25 cm over the past 100 years and much of the rise may be related
to the increase in global mean temperature.

There are inadequate data to determine
whether consistent global changes in climate variability or weather extremes
have occurred over the 20th century. On regional scales there is clear
evidence of changes in some extremes and climate variability indicators
(e.g., fewer frosts in several widespread areas; an increase in the proportion
of rainfall from extreme events over the contiguous states of the USA).
Some of these changes have been toward greater variability; some have been
toward lower variability.

The 1990 to mid­1995 persistent warm­phase
of the El Nino­Southern Oscillation (which causes droughts and floods
in many areas) was unusual in the context of the last 120 years.

4. The balance of evidence
suggests a discernible human influence on global climate

Any human­induced effect on climate
will be superimposed on the background "noise" of natural climate variability,
which results both from internal fluctuations and from external causes
such as solar variability or volcanic eruptions. Detection and attribution
studies attempt to distinguish between anthropogenic and natural influences.
"Detection of change" is the process of demonstrating that an observed
change in climate is highly unusual in a statistical sense, but does not
provide a reason for the change. "Attribution" is the process of establishing
cause and effect relations, including the testing of competing hypotheses.

Since the 1990 IPCC Report, considerable
progress has been made in attempts to distinguish between natural and anthropogenic
influences on climate. This progress has been achieved by including effects
of sulphate aerosols in addition to greenhouse gases, thus leading to more
realistic estimates of human­induced radiative forcing. These have
then been used in climate models to provide more complete simulations of
the human­induced climate­change "signal". In addition, new simulations
with coupled atmosphere­ocean models have provided important information
about decade to century time­scale natural internal climate variability.
A further major area of progress is the shift of focus from studies of
global­mean changes to comparisons of modelled and observed spatial
and temporal patterns of climate change.

The most important results related
to the issues of detection and attribution are:

The limited available evidence from proxy
climate indicators suggests that the 20th century global mean temperature
is at least as warm as any other century since at least 1400 A.D. Data
prior to 1400 are too sparse to allow the reliable estimation of global
mean temperature.

Assessments of the statistical significance
of the observed global mean surface air temperature trend over the last
century have used a variety of new estimates of natural internal and externally­forced
variability. These are derived from instrumental data, palaeodata, simple
and complex climate models, and statistical models fitted to observations.
Most of these studies have detected a significant change and show that
the observed warming trend is unlikely to be entirely natural in origin.

More convincing recent evidence for the
attribution of a human effect on climate is emerging from pattern­based
studies, in which the modelled climate response to combined forcing by
greenhouse gases and anthropogenic sulphate aerosols is compared with observed
geographical, seasonal and vertical patterns of atmospheric temperature
change. These studies show that such pattern correspondences increase with
time, as one would expect, as an anthropogenic signal increases in strength.
Furthermore, the probability is very low that these correspondences could
occur by chance as a result of natural internal variability only. The vertical
patterns of change are also inconsistent with those expected for solar
and volcanic forcing.

Our ability to quantify the human influence
on global climate is currently limited because the expected signal is still
emerging from the noise of natural variability, and because there are uncertainties
in key factors. These include the magnitude and patterns of long­term
natural variability and the time­evolving pattern of forcing by, and
response to, changes in concentrations of greenhouse gases and aerosols,
and land surface changes. Nevertheless, the balance of evidence suggests
that there is a discernible human influence on global climate.

5. Climate is expected
to continue to change in the future

The IPCC has developed a range of scenarios,
IS92a­f, of future greenhouse gas and aerosol precursor emissions based
on assumptions concerning population and economic growth, land­use,
technological changes, energy availability and fuel mix during the period
1990 to 2100. Through understanding of the global carbon cycle and of atmospheric
chemistry, these emissions can be used to project atmospheric concentrations
of greenhouse gases and aerosols and the perturbation of natural radiative
forcing. Climate models can then be used to develop projections of future
climate.

The increasing realism of simulations
of current and past climate by coupled atmosphere­ocean climate models
has increased our confidence in their use for projection of future climate
change. Important uncertainties remain, but these have been taken into
account in the full range of projections of global mean temperature and
sea­level change.

For the mid­range IPCC emission scenario,
IS92a, assuming the "best estimate" value of climate sensitivity4
and including the effects of future increases in aerosol, models project
an increase in global mean surface air temperature relative to 1990 of
about 2°C by 2100. This estimate is approximately one­third lower
than the "best estimate" in 1990. This is due primarily to lower emission
scenarios (particularly for CO2 and the CFCs), the inclusion of the cooling
effect of sulphate aerosols, and improvements in the treatment of the carbon
cycle. Combining the lowest IPCC emission scenario (IS92c) with a "low"
value of climate sensitivity and including the effects of future changes
in aerosol concentrations leads to a projected increase of about 1°C
by 2100. The corresponding projection for the highest IPCC scenario (IS92e)
combined with a "high" value of climate sensitivity gives a warming of
about 3.5°C. In all cases the average rate of warming would probably
be greater than any seen in the last 10,000 years, but the actual annual
to decadal changes would include considerable natural variability. Regional
temperature changes could differ substantially from the global mean value.
Because of the thermal inertia of the oceans, only 50­90% of the eventual
equilibrium temperature change would have been realized by 2100 and temperature
would continue to increase beyond 2100, even if concentrations of greenhouse
gases were stabilized by that time.

Average sea level is expected to rise
as a result of thermal expansion of the oceans and melting of glaciers
and ice­sheets. For the IS92a scenario, assuming the "best estimate"
values of climate sensitivity and of ice­melt sensitivity to warming,
and including the effects of future changes in aerosol, models project
an increase in sea level of about 50 cm from the present to 2100. This
estimate is approximately 25% lower than the "best estimate" in 1990 due
to the lower temperature projection, but also reflecting improvements in
the climate and ice­melt models. Combining the lowest emission scenario
(IS92c) with the "low" climate and ice­melt sensitivities and including
aerosol effects gives a projected sea­level rise of about 15 cm from
the present to 2100. The corresponding projection for the highest emission
scenario (IS92e) combined with "high" climate and ice­melt sensitivities
gives a sea­level rise of about 95 cm from the present to 2100. Sea
level would continue to rise at a similar rate in future centuries beyond
2100, even if concentrations of greenhouse gases were stabilized by that
time, and would continue to do so even beyond the time of stabilization
of global mean temperature. Regional sea­level changes may differ from
the global mean value owing to land movement and ocean current changes.

Confidence is higher in the hemispheric­to­continental
scale projections of coupled atmosphere­ocean climate models than in
the regional projections, where confidence remains low. There is more confidence
in temperature projections than hydrological changes.

All model simulations, whether they were
forced with increased concentrations of greenhouse gases and aerosols or
with increased concentrations of greenhouse gases alone, show the following
features: greater surface warming of the land than of the sea in winter;
a maximum surface warming in high northern latitudes in winter, little
surface warming over the Arctic in summer; an enhanced global mean hydrological
cycle, and increased precipitation and soil moisture in high latitudes
in winter. All these changes are associated with identifiable physical
mechanisms.

In addition, most simulations show a reduction
in the strength of the north Atlantic thermohaline circulation and a widespread
reduction in diurnal range of temperature. These features too can be explained
in terms of identifiable physical mechanisms.

The direct and indirect effects of anthropogenic
aerosols have an important effect on the projections. Generally, the magnitudes
of the temperature and precipitation changes are smaller when aerosol effects
are represented, especially in northern mid­latitudes. Note that the
cooling effect of aerosols is not a simple offset to the warming effect
of greenhouse gases, but significantly affects some of the continental
scale patterns of climate change, most noticeably in the summer hemisphere.
For example, models that consider only the effects of greenhouse gases
generally project an increase in precipitation and soil moisture in the
Asian summer monsoon region, whereas models that include, in addition,
some of the effects of aerosols suggest that monsoon precipitation may
decrease. The spatial and temporal distribution of aerosols greatly influences
regional projections, which are therefore more uncertain.

A general warming is expected to lead
to an increase in the occurrence of extremely hot days and a decrease in
the occurrence of extremely cold days.

Warmer temperatures will lead to a more
vigorous hydrological cycle; this translates into prospects for more severe
droughts and/or floods in some places and less severe droughts and/or floods
in other places. Several models indicate an increase in precipitation intensity,
suggesting a possibility for more extreme rainfall events. Knowledge is
currently insufficient to say whether there will be any changes in the
occurrence or geographical distribution of severe storms, e.g., tropical
cyclones.

Sustained rapid climate change could shift
the competitive balance among species and even lead to forest dieback,
altering the terrestrial uptake and release of carbon. The magnitude is
uncertain, but could be between zero and 200 GtC over the next one to two
centuries, depending on the rate of climate change.

6. There are still many
uncertainties

Many factors currently limit our ability
to project and detect future climate change. In particular, to reduce uncertainties
further work is needed on the following priority topics:

Estimation of future emissions and biogeochemical
cycling (including sources and sinks) of greenhouse gases, aerosols and
aerosol precursors and projections of future concentrations and radiative
properties.

Representation of climate processes in
models, especially feedbacks associated with clouds, oceans, sea ice and
vegetation, in order to improve projections of rates and regional patterns
of climate change.

Future unexpected, large and rapid climate
system changes (as have occurred in the past) are, by their nature, difficult
to predict. This implies that future climate changes may also involve "surprises".
In particular, these arise from the non­linear nature of the climate
system. When rapidly forced, non­linear systems are especially subject
to unexpected behaviour. Progress can be made by investigating non­linear
processes and sub­components of the climatic system. Examples of such
non­linear behaviour include rapid circulation changes in the North
Atlantic and feedbacks associated with terrestrial ecosystem changes.

Footnotes:

1 Climate change
in IPCC Working Group I usage refers to any change in climate over time
whether due to natural variability or as a result of human activity. This
differs from the usage in the UN Framework Convention on Climate Change
where "climate change" refers to a change of climate which is attributed
directly or indirectly to human activity that alters the composition of
the global atmosphere and which is in addition to natural climate variability
observed over comparable time periods.2 A simple
measure of the importance of a potential climate change mechanism. Radiative
forcing is the perturbation to the energy balance of the Earth­atmosphere
system (in Watts per square metre [Wm­2]).3 1 GtC = 1
billion tonnes of carbon.4 In IPCC
reports, climate sensitivity usually refers to the long­term (equilibrium)
change in global mean surface temperature following a doubling of atmospheric
equivalent CO2 concentration. More generally, it refers to the equilibrium
change in surface air temperature following a unit change in radiative
forcing (oC/Wm­2).